1. Field of the Invention
The present invention is related to a light-emitting device produced utilizing nitride semiconductor (InxAlyGa1-x-yN, 0≦x<1, 0≦y<1) and particularly to a nitride semiconductor light-emitting device usable as a high-luminance light source for a backlight of a liquid crystal display device, usual illumination, etc. and also related to a production method thereof.
2. Description of the Background Art
Compound semiconductor light-emitting devices that can emit the three primary color lights of red, green and blue are indispensable in order to utilize the light-emitting devices for various illumination uses. Regarding light-emitting diodes (LEDs), it has not been possible until recent years to utilize LEDs for various illumination uses because the blue LED among LEDs of the three primary colors has not been well-completed and not been available.
However, after the blue LED formed with nitride semiconductor has been developed in the 1990s, illumination products including LEDs are utilized not only for traffic signals but also for backlights in liquid crystal monitors, backlights in liquid crystal televisions and further various illumination uses at home.
Recently, liquid crystal televisions equipped with LED backlights begin to become widely used in a rapid pace in association with their price decline. In addition, illumination devices using LEDs have merits of enabling lower power consumption, smaller space occupied by them, and free of mercury preferably to the environment, as compared with the conventional illumination devices. After the summer of 2009, illumination devices using LEDs have been put on the market at much less prices as compared with those before and thus become popular in a very rapid pace.
In the meantime, light emitted from an illumination device, a backlight of a liquid crystal television, or the like should necessarily be white light. In general, white light obtainable using an LED can be realized by a combination of a blue LED and a yellow YAG (yttrium-aluminum-garnet) phosphor or a combination of a blue LED, a green phosphor and a red phosphor. In other words, a blue LED is needed in the case of obtaining white light utilizing an LED. For this reason, it is desired to provide a method that can produce bright blue LEDs in large amounts at low prices.
In general, III-V compound semiconductors containing nitrogen as a V-group element, such as gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN) and mixed crystals thereof are used for light-emitting layers included in LEDs and laser diodes (LDs) that can emit lights of shorter wavelengths such as blue and bluish green lights.
A usual nitride semiconductor light-emitting device includes an n-type nitride semiconductor layer, a nitride semiconductor light-emitting layer and a p-type nitride semiconductor layer stacked in this order on a sapphire substrate. A p-side electrode pad and an n-side electrode pad for connection to an external electric source are formed on the p-type semiconductor layer side and the n-type semiconductor layer side, respectively.
The sheet resistance of the p-type nitride semiconductor layer is usually higher as compared to that of the n-type nitride semiconductor layer. For the purpose of assisting diffusion of electric current in the p-type semiconductor layer, therefore, a transparent conducive film such as of ITO (Indium Tin Oxide) is stacked on almost the entire area of the p-type semiconductor layer and then the p-side electrode pad is formed on the transparent conductive film. Accordingly, the transparent conductive film transmits light from the light-emitting layer and also acts as a current diffusion layer.
In the meantime, in the case of using an insulative substrate such as a sapphire substrate, it is not allowed to form the n-side electrode pad on the backside of the substrate. Therefore, the n-type semiconductor layer is partly exposed by etching from the p-type semiconductor layer side and thereafter the n-side electrode pad is formed on the exposed area. Then, by supplying electric current between the p-side electric pad and the n-side electrode pad, it is possible to obtain light emission from the light-emitting layer sandwiched between the p-type semiconductor layer and the n-type semiconductor layer.
Regarding the nitride semiconductor light-emitting device having the n-side electrode pad and the p-side electrode pad on the same side of the substrate as described above, each of Japanese National Patent Publication No. 2003-524295 and Japanese Patent Laying-Open No. 2000-164930 teaches to form branch portions extended from the n-side electrode pad and p-side electrode pad thereby to improve current distribution in the light-emitting device.
According to Japanese National Patent Publication No. 2003-524295, p-side branch electrodes are extended from the p-side electrode pad formed on the current diffusion layer made of the transparent conductive film on the p-type semiconductor layer. The light-emitting device has the n-type semiconductor layer including a partial area exposed by etching and then n-side branch electrodes are extended from the n-side electrode pad formed on the exposed area.
The n-side branch electrode and the p-side branch electrode are parallel to each other in their portions opposite to each other. In other ward, it is intended to set constant the distance in which current is diffused from the p-side branch electrode through the current diffusion layer made of the transparent conductive layer. Similarly, it is also intended to set constant the distance in which current is diffused from the n-side branch electrode. With these branch electrodes, therefore, it is possible to improve distribution uniformity of current flowing toward the n-side electrode pad from the p-side electrode pad.
In the meantime, regarding the nitride semiconductor light-emitting device including the transparent conductive film as described above, each of Japanese Patent Laying-Open No. 8-250769 and Japanese Patent Laying-Open No. 2008-192710 teaches to provide an insulative layer just under the electrode pad for the purpose of suppressing current concentration just under the electrode pad.
According to the nitride semiconductor light-emitting device disclosed in each of Japanese Patent Laying-Open No. 8-250769 and Japanese Patent Laying-Open No. 2008-192710, the insulative layer provided just under the electrode pad can effectively promotes current diffusion in the lateral directions in the transparent conductive film, thereby improving the light emission efficiency. However, there are problems as follows: light emission is intensified in the vicinity of the n-side electrode pad; good electric properties cannot be obtained; and the light emission efficiency cannot necessarily be improved.
Under the circumstances, Japanese Patent Laying-Open No. 2010-232649 for example discloses a nitride semiconductor light-emitting device, intending to obtain the effects as follows: current concentration in the transparent conductive film and the semiconductor layer just under the p-side electrode pad is suppressed thereby improving the light emission efficiency; light loss due to absorption and multiple-reflection caused by the electrode is suppressed thereby improving the light extraction efficiency; and the external quantum efficiency and electric properties are improved.
Regarding the conventional nitride semiconductor light-emitting device, it has been considered that from the viewpoint of the light emission efficiency, it is preferable to set the sheet resistance values of the n-side and p-side layers to be approximately the same thereby uniformly diffusing the current. On the other hand, Japanese Patent Laying-Open No. 2010-232649 describes that, as shown in FIGS. 9(a) and (b), the sheet resistance of n-type nitride semiconductor layer 101 on the n-side is set lower than that of transparent conductive film 102 on the p-side, thereby reducing light emission in the vicinity of n-side electrode pad 103 and then improving the light extraction efficiency. In this case, it is also described that the light absorption and multiple-reflection due to p-side electrode pad 104 can be prevented by providing insulative layer 105 just under the electrode pad.
Japanese Patent Laying-Open No. 2010-232649 further describes that the operation voltage of nitride semiconductor light-emitting device 100 is influenced mainly by the sheet resistance of the n-side layers and then it is possible to significantly improve the electric properties by reducing the sheet resistance of the n-side layers, particularly in the case that high current of about 30-100 mA is applied to the light-emitting device.
In recent years, it is demanded that the nitride semiconductor light-emitting devices having further improved properties (higher output, lower voltage, lower heat emission) are provided at lower costs and then the devices having sheet resistance values of the n-side and p-side less than 10Ω/□ have been developed.
In such a nitride semiconductor light-emitting device, it is difficult to further reduce the sheet resistance value. If n-type impurities are added for the purpose of further reducing the sheet resistance value of the n-type nitride semiconductor layer, there is caused a problem that the crystalline quality of the n-type nitride semiconductor layer is deteriorated. Further, if the thickness of the n-type nitride semiconductor layer is increased, there is caused a problem that the productivity is lowered and the costs are increased.